Field of the Invention
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine.
Description of the Related Art
There has been known a method of diagnosing deterioration of a three-way catalyst (a so-called Cmax method), by performing active control to switch the air fuel ratio of exhaust gas between a rich air fuel ratio and a lean air fuel ratio in an alternate manner, and measuring a maximum storage amount of oxygen (Cmax) stored in the three-way catalyst (for example, refer to a first patent literature). Here, the maximum storage amount of oxygen in the three-way catalyst is related to a period of time and an air fuel ratio of exhaust gas, from the time when the air fuel ratio of exhaust gas flowing into the three-way catalyst has changed from a rich air fuel ratio to a lean air fuel ratio until the time when the air fuel ratio of exhaust gas flowing out of the three-way catalyst changes to a lean air fuel ratio. Similarly, the maximum storage amount of oxygen in the three-way catalyst is also related to a period of time and an air fuel ratio of exhaust gas, from the time when the air fuel ratio of exhaust gas flowing into the three-way catalyst has changed from the lean air fuel ratio to the rich air fuel ratio until the time when the air fuel ratio of exhaust gas flowing out of the three-way catalyst changes to a rich air fuel ratio. Accordingly, the maximum storage amount of oxygen in the three-way catalyst can be calculated based on these periods of time and air fuel ratios.
According to this Cmax method, for example, in cases where the maximum storage amount of oxygen in the three-way catalyst becomes smaller than a threshold value, a determination is made that the three-way catalyst has deteriorated. This deterioration is unrecoverable deterioration, and is thermal deterioration, for example.